Apple will reportedly release an update later this year that'll let you use third-party messaging apps with Siri as the default. According to Bloomberg, Siri will decide which messaging service to use based on your interactions with specific contacts. So if you always message your friend Brad using WhatsApp, saying "Hey Siri, message Brad" will cause Siri to launch WhatsApp rather than iMessage.
Currently, if you want to message someone through a third-party app using Siri, you need to specify which service you'd like to use. This impending update will allegedly drop that requirement.
It's also said that Siri will soon let you use third-party calling apps as the default, too, but there's no mention on exactly when that functionality will roll out.
Having to constantly specify which messaging app to use when you're talking to Siri could get annoying, and it could deter people from actually using Siri to message their friends. This update to allow Siri to default to third-party messaging apps sounds like it'll change that and make Siri easier and better to use. While it doesn't sound like there will just be a toggle for you to flip for using a different third-party messaging app, this update is still an improvement, and hopefully Siri will learn which third-party apps to default to quickly.